1. Field of the Invention
The present invention relates to a recording apparatus, and more specifically, it relates to a recording apparatus having a unit that controls turning on and off of the power supply to a recording head.
2. Description of the Related Art
Current ink jet recording apparatuses have an ink jet head having many ink jet nozzles and select nozzles to eject ink according to print data. In such recording apparatuses, the number of nozzles to drive at the same time varies depending on print data, and the power consumed by the recording head also vary greatly. For this reason, the instantaneous power consumption of the recording head is considerably large compared to the average power consumption thereof. Of course, a power source that supplies power to the recording head needs to have a capacity to supply power that exceeds the power consumption of the recording head.
Recent recording apparatuses tend to have an increased number of nozzles and an increased driving speed to achieve faster and finer recording and also tend to consume increased power during operation. It is not unusual for common and inexpensive ink jet printers for home use to instantaneously consume power exceeding 20 W during operation, so the power source must have sufficient capacity therefor.
In order to stably drive the recording head so that the ejection characteristics do not vary depending on the number of nozzles to drive at the same time, the power source must have a low-impedance output characteristic such that the power source voltage varies little when the electrical load varies.
In power source circuits used in relatively inexpensive electronic devices such as household appliances, inserting a resistor into a route through which the current flows and measuring the voltage drop due to the resistor is a usual method for detecting the change in load current of the power source output. However, when a low-impedance power source such that the power source voltage varies little when the electrical load varies is required, it is undesirable to insert a resistance component into the electrical route. In order to detect the load current of the power source output, an expensive detecting circuit is necessary. This increases the cost of the device.
In some known recording apparatuses, when the recording head is not attached to the main body of the recording apparatus, the power supply to the recording head is stopped. Such recording apparatuses have a unit that controls turning on and off of the power supply to the recording head and thereby perform control so as not to apply unnecessary load current to the head, the unnecessary load current being, for example, due to the short circuit when the recording head is attached.
However, since the recording apparatuses have a unit that controls turning on and off of the power supply to the recording head, an inrush current to the recording head can occur when the power supply is turned on after the head is attached. In this inrush current, a large current flows in a short period of time. Therefore, electrical noise is generated and the circuit malfunctions. Recent FETs have low on-resistance and high response speed and exhibit excellent characteristics as switching elements. As shown in FIG. 3 of U.S. Pat. No. 5,711,619, there is known a configuration in which a FET is used as an element for switching the power supply to the recording head. Such a configuration increases an inrush current when the switching characteristic is on, and a malfunction due to noise can occur.
In the configuration disclosed in U.S. Pat. No. 5,711,619, a unit that detects whether or not the recording head is attached is provided, and when the recording head is not attached, the power supply to the recording head is stopped.
However, when the recording head is attached and the power supply to the recording head is turned on, if a problem due to trouble or deterioration of the recording head, such as an electrical short circuit of the recording head itself, occurs, it is impossible to detect such a problem and to stop the power supply to the recording head.
In such a case, a short circuit occurs between the power source voltage HVH to the recording head and the head (including the inside of the head), and the resistance value in the circuit in the head becomes zero to several ohms. In such a short circuit state, the power source becomes overloaded in a short time, and therefore the overload protection function of the power source works to shut down the power source. Since the power supply is stopped in a short time, problems such as temperature rise are unlikely to occur.
In contrast, a halfway short circuit such that the resistance value of the circuit in the head becomes several to several hundred ohms, can occur. In such a short circuit state, the overcurrent in the head generated by the short circuit is small, and about the same amount of current as the current that flows during normal head driving, continues to flow. That is, due to the abnormal current flow, about several watts of additional power is consumed. Considering the power supplying capability of the power source, if the power increases by about several watts, the recording head can continue to operate without problems. However, even though it is about several watts, if abnormal power is consumed for many hours, the temperature rises and secondary effects on other components can occur.